Polarization dispersion compensating apparatus

ABSTRACT

A polarization dispersion compensation apparatus includes a polarization controller, a polarization beam splitter, an optical delay circuit, and a polarization beam combiner. The polarization controller controls polarization of an optical signal so that the polarization axis of the input optical signal substantially coincides with the optical axis of an optical transmission line, and the polarization beam splitter section splits the optical signal into two polarized components perpendicular to each other. The optical delay circuit section causes a difference in delay between the two polarized components, and the polarization beam combiner section combines the two polarized components output from the optical delay circuit section. Each of the polarization beam splitter section and the polarization beam combiner section includes a symmetric Mach-Zehnder interferometer having optical transmission lines in two arms, each arm including a temperature control and a birefringence portion for compensating polarization dispersion between the two polarized components.

TECHNICAL FIELD

The present invention relates to a polarization dispersion compensationapparatus for compensating polarization dispersion, which may become afactor in restricting transmission rate or transmission distance in anoptical fiber cable for use in a superfast optical communicationssystem.

BACKGROUND ART

In the optical fiber cable for use in a superfast optical communicationssystem operating at 40 Gbps or more, comparatively large polarizationdispersion occurs, and this becomes a factor for restrictingtransmission rate or transmission distance. This polarization dispersionoccurs as follows. As shown in FIG. 23, degeneration of the base modeoccurs due to decentering of a core of an optical fiber cable 100 andapplication of a non-axisymmetric stress to the core. A group delaydifference occurs between a TE wave and a TM wave depending on adifference in the propagation velocity between the optical signals ofthe two polarized components of the TE wave and the TM wave, which areperpendicular to each other. As a result, broadening occurs in thetemporal direction in an optical pulse signal, restricting transmissionrate and transmission distance in the communication system. In order tosolve the above-mentioned problems, it is required to compensatepolarization so that no group delay difference occurs between the twopolarized components by controlling the polarization state in thereception terminal station and generating a group delay differenceinverse to the group delay difference that occurs in the optical fibercable.

FIG. 24 is a perspective view showing polarization compensation with apolarization-maintaining optical fiber cable 101 of prior art, and FIG.25 is a longitudinal sectional view showing a construction of thepolarization-maintaining optical fiber cable 101 of FIG. 24.

The prior art example shown in FIG. 24 compensates for the polarizationdispersion using the polarization-maintaining optical fiber cable 101.As shown in FIG. 25, this polarization-maintaining optical fiber cable101 has a predetermined birefringence with a non-axisymmetric stressapplied to a core 102 by inserting silica glasses 104 and 105 doped withBa₂O₃ on both sides of the core 102 in the portion of a cladding 103around the core 102. In this polarization-maintaining optical fibercable 101, the propagation velocities of the polarized componentsperpendicular to each other are different from each other. Therefore, tothe polarization dispersion of the optical fiber cable can becompensated on this basis. In practice, to match the polarization axisof incident light to the polarization axis of thepolarization-maintaining optical fiber cable 101, it is required toprovide a polarization controller including a half-wavelength plate anda quarter-wavelength plate at the preceding stage of thepolarization-maintaining optical fiber cable 101.

FIG. 26 is a plan view showing a construction of the polarizationdispersion compensation apparatus disclosed in FIG. 3 of a first priorart document, “Teruhiko Kudou et al., Theoretical Basis of PolarizationMode Dispersion Equalization up to the Second Order, Journal ofLightwave Technology, Vol. 18, No. 4, pp. 614-617, April 2000”. Asdescribed above, the polarization dispersion changes depending onenvironmental changes in the optical fiber cable, and accordingly, thereis needed a polarization dispersion compensator capable of adjusting apolarization dispersion value.

In the prior art example of FIG. 26, a polarization beam splitter 111 isprovided at the final stage of an optical transmission line 110 forpropagating an inputted optical signal, and this is followed by aconstruction provided with two variable phase shifters PS1 and PS2, adirectional coupler DC101, two variable phase shifters PS3 and PS4 andtwo variable optical delay circuits 113 and 114 as well as apolarization beam combiner 112 further provided at the final stage ofthe polarization dispersion compensation apparatus. In the polarizationdispersion compensation apparatus of this prior art, compensation ismade for the polarization dispersion by varying the polarizationdispersion by means of an optical processing circuit. However, thisprior art example has had the problem that the size of the apparatus hasbeen comparatively large and the transmission loss has been increaseddue to the use of the bulk type polarization beam splitter and thepolarization beam combiner.

FIG. 27 is a plan view showing a construction of the polarizationdispersion compensation apparatus disclosed in the specification of U.S.Pat. No. 5,930,414, which is a second prior art document.

The prior art of FIG. 27 is provided with a polarization controller 120constituted by including a quarter-wavelength plate 121 and ahalf-wavelength plate 122 provided at the first stage, a polarizationbeam splitter 123 for splitting an inputted optical signal into twopolarized components at the next stage and a polarization beam combiner124 for combining these two polarized components at the final stage.Moreover, in order to cause a variable delay time between the twopolarized components, a plurality of asymmetrical Mach-Zehnderinterferometers 130 to 132, which are constituted by including opticalwaveguides and connected in concatenation, are provided between thepolarization beam splitter 123 and the polarization beam combiner 124via directional couplers 141 to 143, which have adjustable couplingcoefficients. Further, it is required to switch the relative opticalphase in the Mach-Zehnder interferometers 130 to 132 with respect to thestructural common-mode interference of the two optical signals, whichare outputted from two arm portions of the Mach-Zehnder interferometers130 to 132 and thereafter inputted to the directional couplers 141 to143 at the subsequent stages. For the above reasons, the Mach-Zehnderinterferometers 130 to 132 are provided with variable phase shifters 150to 152, respectively. Therefore, in this prior art example, compensationis made for the polarization dispersion by combining the directionalcouplers 140 to 143 with an optical processing circuit that has aMach-Zehnder structure. However, even this prior art example has had theproblem that the size of the apparatus had become comparatively large.Therefore, the polarization dispersion compensator is required to have alower loss and a small size as well as a high operating speed and a lowconsumption of power.

FIG. 28 is a plan view showing a construction of the polarizationdispersion compensation apparatus disclosed in a third prior artdocument, “Japanese Patent Laid-Open Publication No. 2001-42272”.

In the prior art of FIG. 28, an input channel optical waveguide 211, anoptical waveguide type polarization beam splitter element 212, a pair ofoptical waveguides 213 a and 213 b, a variable branching ratio opticalcoupler 214, a pair of optical delay lines 215 a and 215 b, an opticalwaveguide type polarization beam combiner element 216 and an outputchannel optical waveguide 217 a are successively formed on a siliconsubstrate 210. Further, polarization change means 218 a and 218 b areformed in either one of a pair of optical waveguides 213 a and 213 b andin either one of a pair of optical delay lines 215 a and 215 b.Moreover, phase adjustment means 219 a and 219 b for adjusting therelative phase difference are formed in a pair of optical waveguides 213a and 213 b. In this case, one input port of the optical waveguide typepolarization beam splitter element 212 is optically connected with theinput channel optical waveguide 211, while two input ports of thevariable branching ratio optical coupler 214 are optically connectedwith the two output ports of the optical waveguide type polarizationbeam splitter element 212 via a pair of optical waveguides 213 a and 213b. Moreover, a pair of optical delay lines 215 a and 215 b has one endoptically connected with two output ports of the variable branchingratio optical coupler 214 and the other end optically connected with twoinput ports of the optical waveguide type polarization beam combinerelement 216. One output port of the optical waveguide type polarizationbeam combiner element 216 is optically connected with the output channeloptical waveguide 217 a.

In the polarization dispersion compensation apparatus constituted asabove, the phase adjustment means 219 a and 219 b and the variablebranching ratio optical coupler 214 operate as a polarization controller300, and the optical delay lines 215 a and 215 b operate as a variabledelay line, constituting the polarization dispersion compensationcircuit as a whole. With this arrangement, this polarization dispersioncompensation circuit can compensate for the primary polarizationdispersion in the optical transmission line.

In this polarization dispersion compensation apparatus, the polarizationcontroller 300 exists between the polarization beam splitter element 212and the optical delay lines 215 a and 215 b. An optical signal inputtedto the polarization beam splitter element 212 is split into the TE waveand the TM wave. Subsequently, the optical signal has a polarizationcontrolled by the polarization controller 300 constituted by includingthe phase adjustment means 219 a and the variable branching ratiocoupler 214, and thereafter, the controlled optical signal is inputtedto a pair of optical delay lines 215 a and 215 b. The amount of delay ofthe optical signal is adjusted by the optical delay lines 215 a and 215b, and the polarization dispersion of the optical signal is compensated.However, in this polarization dispersion compensation apparatus controlof compensator of the polarization dispersion is very difficult since itis required to adjust the three points of the phase adjustment means 219a and 219 b, the variable branching ratio optical coupler 214, and theoptical delay lines 215 a and 215 b.

It is an object of the present to solve the aforementioned problems andprovide a polarization dispersion compensation apparatus, which has asmaller size and a lighter weight than the prior art, and is able tocompensate for the polarization dispersion with lower loss.

Another object of the present invention is to solve the aforementionedproblems and provide a polarization dispersion compensation apparatus,which is able to control compensation for a polarization dispersion moreeasily than in the prior art.

DISCLOSURE OF THE INVENTION

According to one aspect of the present invention, there is provided apolarization dispersion compensation apparatus including:

polarization control means for controlling a polarization state of aninputted optical signal so that a polarization axis of the opticalsignal substantially coincides with an optical axis of an opticaltransmission line;

polarization beam splitter means for splitting an optical signaloutputted from the polarization control means, and outputting opticalsignals of two polarized components perpendicular to each other;

optical delay means including two optical transmission lines of lengthsdifferent from each other, and causing a difference in delay between thetwo polarized components of the optical signal outputted from thepolarization beam splitter means; and

polarization beam combiner means for combining the two polarizedcomponents of the optical signal outputted from the optical delay means,and outputting a combined optical signal,

wherein the polarization beam splitter means includes a symmetricMach-Zehnder interferometer having optical transmission lines of firstand second arm portions, and at least one of the first and second armportions includes first refractive index control means for controlling arefractive index of the optical signal propagating through the opticaltransmission line of the arm portion and first birefringence means forcausing birefringence in the optical signal propagating through theoptical transmission line of the arm portion,

wherein the polarization beam combiner means includes a symmetricMach-Zehnder interferometer having optical transmission lines of thirdand fourth arm portions, and at least one of the third and fourth armportions includes second refractive index control means for controllinga refractive index of the optical signal propagating through the opticaltransmission line of the arm portion and second birefringence means forcausing birefringence in the optical signal propagating through theoptical transmission line of the arm portion.

In the above-mentioned polarization compensation apparatus, thepolarization beam splitter means includes:

a first directional coupler for distributing the inputted optical signalinto two optical signals, and outputting the distributed two opticalsignals;

the optical transmission line of the first arm portion for propagatingone optical signal out of the two optical signals distributed by thefirst directional coupler;

the optical transmission line of the second arm portion for propagatingthe other optical signal out of the two optical signals distributed bythe first directional coupler; and

a second directional coupler for combining the optical signalpropagating through the optical transmission line of the first armportion with the optical signal propagating through the opticaltransmission line of the second arm portion and for thereafterdistributing a resulting combined optical signal into two opticalsignals, and outputting the distributed two optical signals, and

wherein the polarization beam combiner means includes:

a third directional coupler for combining the inputted two opticalsignals, and for thereafter distributing a resulting combined opticalsignal into two optical signals, and outputting the distributed twooptical signals;

the optical transmission line of the third arm portion for propagatingone optical signal out of the two optical signals distributed by thethird directional coupler;

the optical transmission line of the fourth arm portion for propagatingthe other optical signal out of the two optical signals distributed bythe third directional coupler; and

a fourth directional coupler for combining the optical signalpropagating through the optical transmission line of the third armportion with the optical signal propagating through the opticaltransmission line of the fourth arm portion and for thereafteroutputting a resulting combined optical signal.

Also, in the above-mentioned polarization dispersion compensationapparatus, the first refractive index control means controls atemperature of the optical transmission line of the arm portion providedwith the first refractive index control means, so as to control therefractive index of the optical signal propagating through the opticaltransmission line of the arm portion, and

wherein the second refractive index control means controls a temperatureof the optical transmission line of the arm portion provided with thesecond refractive index control means, so as to control the refractiveindex of the optical signal propagating through the optical transmissionline of the arm portion.

Further, in the above-mentioned polarization dispersion compensationapparatus, the first refractive index control means controls an electricfield applied to the optical transmission line of the arm portion whichis subjected to a predetermined polling process and which is providedwith the first refractive index control means, so as to control therefractive index of the optical signal propagating through the opticaltransmission line of the arm portion, and

wherein the second refractive index control means controls an electricfield applied to the optical transmission line of the arm portion whichis subjected to a predetermined polling process and which is providedwith the second refractive index control means, so as to control therefractive index of the optical signal propagating through the opticaltransmission line of the arm portion.

Furthermore, in the above-mentioned polarization dispersion compensationapparatus, the first birefringence means irradiates the opticaltransmission line of the arm portion provided with the firstbirefringence means with ultraviolet rays, so as to cause birefringencein the optical signal propagating through the optical transmission lineof the arm portion, and

wherein the second birefringence means irradiates the opticaltransmission line of the arm portion provided with the secondbirefringence means with ultraviolet rays, so as to cause birefringencein the optical signal propagating through the optical transmission lineof the arm portion.

Also, the above-mentioned polarization dispersion compensation apparatusfurther includes third refractive index control means for controlling atemperature of one optical transmission line out of the two opticaltransmission lines of the optical delay means, so as to control therefractive index of the optical signal propagating through the oneoptical transmission line.

Further, in the above-mentioned polarization dispersion compensationapparatus, one optical transmission line out of the two opticaltransmission lines of the optical delay means is subjected to apredetermined polling process, and

wherein the polarization dispersion compensation apparatus furtherincludes fourth refractive index control means controls an electricfield applied to the one optical transmission line subjected to thepolling process, so as to control the refractive index of the opticalsignal propagating through the one optical transmission line.

Also, in the above-mentioned polarization dispersion compensationapparatus, the optical transmission line is an optical waveguide formedon a substrate.

Further, in the above-mentioned polarization dispersion compensationapparatus, the optical transmission line is an optical fiber cable.

Also, according to another aspect of the present invention, there isprovided a polarization dispersion compensation apparatus including:

polarization control means for controlling a polarization state of aninputted optical signal so that a polarization axis of the opticalsignal substantially coincides with an optical axis of an opticaltransmission line;

polarization beam splitter means for splitting an optical signaloutputted from the polarization control means, and outputting opticalsignals of two polarized components perpendicular to each other;

optical delay means including two optical transmission lines of lengthsdifferent from each other, and causing a difference in delay between thetwo polarized components of the optical signal outputted from thepolarization beam splitter means; and

polarization beam combiner means for combining the two polarizedcomponents of an optical signal outputted from the optical delay means,and outputting a resulting combined optical signal,

wherein the polarization beam splitter means includes a fifthdirectional coupler having mutually adjacent two optical transmissionlines, distributing the inputted optical signal into two opticalsignals, and outputting the distributed two optical signals,

wherein the mutually adjacent two optical transmission lines of thefifth directional coupler includes fourth refractive index control meansfor controlling a refractive index of optical signals propagatingthrough the two optical transmission lines and third birefringence meansfor causing birefringence in the optical signals propagating through thetwo optical transmission lines,

wherein the polarization beam combiner means includes a sixthdirectional coupler having mutually adjacent two optical transmissionlines, distributing the inputted optical signal into two opticalsignals, and outputting the distributed two optical signals, and

wherein the mutually adjacent two optical transmission lines of thesixth directional coupler includes fifth refractive index control meansfor controlling a refractive index of optical signals propagatingthrough the two optical transmission lines and fourth birefringencemeans for causing birefringence in the optical signals propagatingthrough the two optical transmission lines.

Further, according to a further aspect of the present invention, thereis provided a polarization dispersion compensation apparatus including:

polarization beam splitter means for splitting and outputting opticalsignals of two polarized components perpendicular to each other;

polarization control means including phase adjustment means and avariable branching ratio coupler and controlling a polarization state ofthe optical signal;

a pair of optical delay means for delaying the optical signal after thepolarization beam splitting; and

polarization beam combiner means having first and second output ports,combining the two polarized components of the optical signal outputtedfrom the optical delay means, and outputting a resulting combinedoptical signal through the first output port,

wherein the polarization beam splitter means and the polarization beamcombiner means is of a symmetric Mach-Zehnder interferometer havingbirefringence at least in one arm portion, and

wherein the phase adjustment means and the variable branching ratiocoupler is controlled so that a level of a signal outputted from thefirst output port of the polarization beam combiner means becomes themaximum or so that a level of a signal outputted from the second outputport of the polarization beam combiner means becomes the minimum.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to a firstpreferred embodiment of the present invention;

FIG. 2 is a plan view showing a detailed construction of a polarizationbeam splitter section 1 of FIG. 1;

FIG. 3 is a graph showing an intensity of an optical output signal atoutput ports P23 and P24 of the polarization beam splitter section 1with respect to a refractive index change Δn due to a birefringenceportion BF1 of FIG. 1;

FIG. 4 is a graph showing an intensity of the optical signal that passesthrough an optical waveguide 21 located just behind the output port P23with respect to an applied electric power of a temperature controlportion HT1 of FIG. 1;

FIG. 5 is a graph showing a group delay time difference [psec] betweenthe TE wave and the TM wave with respect to the applied electric powerof the temperature control portion HT1 of FIG. 1;

FIG. 6 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to asecond preferred embodiment of the present invention;

FIG. 7 is a plan view showing a detailed construction of a polarizationcontroller 4 a of FIG. 6;

FIG. 8 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to a thirdpreferred embodiment of the present invention;

FIG. 9 is a plan view showing a detailed construction of an electricfield control portion EF1 of FIG. 8;

FIG. 10 is a longitudinal sectional view showing a detailed constructionof the electric field control portion EF1 of FIG. 8;

FIG. 11 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to afourth preferred embodiment of the present invention;

FIG. 12 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to a fifthpreferred embodiment of the present invention;

FIG. 13 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to a sixthpreferred embodiment of the present invention;

FIG. 14 is a plan view showing a construction of an optical fiber cabletype polarization dispersion compensation apparatus according to aseventh preferred embodiment of the present invention;

FIG. 15 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to aneighth preferred embodiment of the present invention;

FIG. 16 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to a ninthpreferred embodiment of the present invention;

FIG. 17 is a plan view showing a construction of part of a polarizationbeam splitter section 1 for an optical waveguide type polarizationdispersion compensation apparatus according to a first modifiedpreferred embodiment of the present invention;

FIG. 18 is a plan view showing a construction of part of a polarizationbeam splitter section 1 for an optical waveguide type polarizationdispersion compensation apparatus according to a second modifiedpreferred embodiment of the present invention;

FIG. 19 is a plan view showing a construction of part of a polarizationbeam splitter section 1 for an optical waveguide type polarizationdispersion compensation apparatus according to a third modifiedpreferred embodiment of the present invention;

FIG. 20 is a plan view showing a construction of part of a polarizationbeam combiner section 3 for an optical waveguide type polarizationdispersion compensation apparatus according to a fourth modifiedpreferred embodiment of the present invention;

FIG. 21 is a plan view showing a construction of part of a polarizationbeam combiner section 3 for an optical waveguide type polarizationdispersion compensation apparatus according to a fifth modifiedpreferred embodiment of the present invention;

FIG. 22 is a plan view showing a construction of part of a polarizationbeam combiner section 3 for an optical waveguide type polarizationdispersion compensation apparatus according to a sixth modifiedpreferred embodiment of the present invention;

FIG. 23 is a perspective view showing polarization dispersion in anoptical fiber cable 100 for transmission according to a prior art;

FIG. 24 is a perspective view showing polarization compensation with apolarization-maintaining optical fiber cable 101 according to a priorart;

FIG. 25 is a longitudinal sectional view showing a construction of thepolarization-maintaining optical fiber cable 101 of FIG. 24;

FIG. 26 is a plan view showing a construction of the polarizationdispersion compensation apparatus disclosed in a first prior artdocument;

FIG. 27 is a plan view showing a construction of the polarizationdispersion compensation apparatus disclosed in a second prior artdocument; and

FIG. 28 is a plan view showing a construction of the polarizationdispersion compensation apparatus disclosed in a third prior artdocument.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings. Components similar to thoseshown in the following figures are denoted by the same referencenumerals.

First Preferred Embodiment

FIG. 1 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to thefirst preferred embodiment of the present invention. The polarizationdispersion compensation apparatus of this preferred embodiment isconstituted by including a polarization controller 4, a polarizationbeam splitter section 1, an optical delay circuit section 2 and apolarization beam combiner section 3, which are cascaded. Each of thepolarization beam splitter section 1 and the polarization beam combinersection 3 has a symmetric Mach-Zehnder interferometer having two armportions. In the polarization beam splitter section 1, one arm portionout of the two arm portions is provided with a temperature controlportion HT1 for controlling the refractive index of the optical signalthat propagates through its optical waveguide 21, and the other armportion is provided with a birefringence portion BF1 for causingbirefringence in the optical signal that propagates through its opticalwaveguide 22. In the polarization beam combiner section 3, one armportion out of the two arm portions is provided with a temperaturecontrol portion HT2 for controlling the refractive index of the opticalsignal that propagates through its optical waveguide 21, and the otherarm portion is provided with a birefringence portion BF2 for causingbirefringence in the optical signal that propagates through its opticalwaveguide 22.

Referring to FIG. 1, an optical fiber cable 20 for propagating theinputted optical signal is provided with the polarization controller 4which has been known to those skilled in the art, and this polarizationcontroller 4 is provided with a quarter-wavelength plate 41 and ahalf-wavelength plate 42. By adjusting the levels of a control signalSc1 applied to the quarter-wavelength plate 41 and a control signal Sc2applied to the half-wavelength plate 42, the polarization rotation angleof the optical signals that propagate through the wavelength plates 41and 42 is adjusted, and this leads to that the polarization state of theoptical signals is controlled so that the polarization axis of theoptical signal that propagates through the optical fiber cable 20substantially coincides with the optical axis of the optical waveguide21 formed on an optical waveguide substrate 10. Then, the terminalportion of the optical fiber cable 20 is connected with the opticalwaveguide 21 formed on the optical waveguide substrate 10 via an opticalfiber connector 20C.

With regard to the optical waveguide substrate 10, a core is formed byforming a quartz film as an undercladding on a silicon substrate by theCVD method and thereafter doping the quartz film with a predeterminedimpurity. Subsequently, an overcladding is formed, and two opticalwaveguides 21 and 22 are formed. In this case, the two opticalwaveguides 21 and 22 are formed to be adjacent to each other so that theoptical signals are combined in four portions, and these four portionsbecome 3-dB directional couplers DC1, DC2, DC3 and DC4. In this case,the directional coupler DC1 has two input ports P11 and P12 and twooutput ports P13 and P14, and the directional coupler DC2 has two inputports P21 and P22 and two output ports P23 and P24. The directionalcoupler DC3 has two input ports P31 and P32 and two output ports P33 andP34, and the directional coupler DC4 has two input ports P41 and P42 andtwo output ports P43 and P44.

Two arm portions are formed of the two optical waveguides 21 and 22between the two directional couplers DC1 and DC2, two arm portions areformed of the two optical waveguides 21 and 22 between the twodirectional couplers DC2 and DC3, and two arm portions are formed of thetwo optical waveguides 21 and 22 between the two directional couplersDC3 and DC4. A resistive terminator 91 is connected with the input portP12 of the directional coupler DC1, and a resistive terminator 92 isconnected with the output port P44 of the directional coupler DC4. Theoptical waveguide 21 has an input terminal denoted by a referencenumeral P10 and an output terminal denoted by a reference numeral P50.

The polarization beam splitter section 1 is constituted by including thedirectional coupler DC1, the temperature control portion HT1 providedwith a heater formed on the optical waveguide 21 that is one armportion, the birefringence portion BF1 formed on the optical waveguide22 that is the other arm portion and the directional coupler DC2. Thetemperature control portion HT1 is provided to control the refractiveindex of the optical signal that propagates through the opticalwaveguide 21 by controlling the temperature of the optical waveguide 21by heating. The birefringence portion BF1 is formed by irradiating theoptical waveguide 22 of the other arm portion, with ultraviolet rayshaving an energy of 50 mJ/cm² and a wavelength of 193 nm after beingsubjected to pulse intensity modulation at a frequency of 60 Hz for fiveminutes by means of an ArF laser apparatus 50 provided with an ArFexcimer laser. This birefringence portion BF1 causes birefringence inthe optical signal that propagates through the optical waveguide 22,which is the other arm portion.

The polarization beam splitter section 1 having the symmetricMach-Zehnder interferometer is constituted by including the temperaturecontrol portion HT1 and the birefringence portion BF1 formed as above,and the principle of this operation will be described in detail below.FIG. 2 shows a detailed construction of the polarization beam splittersection 1. Since the birefringence portion BF1 is formed in the opticalwaveguide 22 of the other arm portion, the inputted optical signal canbe split into a TE wave and a TM wave, which are polarized componentsperpendicular to each other, by the polarization beam splitter section1.

FIG. 3 is a graph showing an intensity of an optical output signal atthe output ports P23 and P24 of the polarization beam splitter section 1with respect to a refractive index change An due to the birefringenceportion BF1 of FIG. 1. FIG. 4 is a graph showing an intensity of theoptical signal that passes through the optical waveguide 21 located justbehind the output port P23 with respect to the applied electric power ofthe temperature control portion HT1 of FIG. 1. If the refractive indexin one arm portion changes in the polarization beam splitter section 1that has the symmetric Mach-Zehnder interferometer, then the phase ofthe optical signal that propagates through the optical waveguide 22changes, and the optical signal from the output ports P23 and P24changes in intensity as shown in FIG. 3. The fact that birefringenceoccurs in one arm portion means that the refractive index change variesdepending on the TE wave and the TM wave. Accordingly, a refractiveindex change Δn₁ possibly occurs in the TE wave at the maximum point ofthe TE wave, and a refractive index change Δn₂ possibly occurs in the TMwave at the maximum point of the TM wave. Therefore, as shown in FIG. 4,by changing the refractive index of the optical signal with the appliedelectric power of the temperature control portion HT1 adjusted, the TEwave is outputted from the output port P23, while the TM wave isoutputted from the output port P24.

The optical delay circuit section 2 of FIG. 1 is constituted byincluding optical waveguides 21 and 22, which have lengths differentfrom each other. The length of the optical waveguide 21 is set to belonger than the length of the optical waveguide 22, and the TE wave thatpropagates through the optical waveguide 21 is delayed in the temporaldirection as compared with the TM wave that propagates through theoptical waveguide 22. Due to the difference in the delay, polarizationdispersion occurs between the TE wave and the TM wave.

Further, the polarization beam combiner section 3 is formed by a formingprocess similar to that of the polarization beam splitter section 1 andis provided with the directional coupler DC3, the temperature controlportion HT2 provided with the heater formed on the optical waveguide 21that is one arm portion, the birefringence portion BF2 formed on theoptical waveguide 22 that is the other arm portion, and the directionalcoupler DC4.

At the output terminal of this polarization dispersion compensationapparatus is provided a polarization beam combiner section 3, which hasthe same construction as that of the polarization beam splitter section1. With the construction of the apparatus, the refractive indexes of theoptical signals that propagate through the optical waveguides 21 and 22change by changing the electric power applied to the temperature controlportion HT1, with the result that the propagation paths of the TE waveand the TM wave change as shown in FIG. 4, allowing the propagationpaths of the polarized waves to be changed. As a result, a group delaytime difference [psec] (corresponding to the polarization dispersionvalue) between the TE wave and the TM wave can be controlled as shown inFIG. 5. Further, by adjustment of the electric power applied to thetemperature control portion HT2 of the polarization beam combinersection 3 in a manner similar to that of the adjustment of the electricpower applied to the temperature control portion HT1 of the polarizationbeam splitter section 1, an optical signal can be controlled andconsistently output from the output port P43.

As described above, the polarization dispersion between the TE wave andthe TM wave can be adjusted in accordance with the environmental changeof the optical fiber cable for transmission connected at the precedingstage of this polarization dispersion compensation apparatus so as toconsistently make compensation by adjusting the temperature controlportions HT1 and HT2 in a similar manner.

As compared with the prior art example, the elements, which constitutethe apparatus, can be downsized and made to have a lower loss with theconstruction in which the optical waveguides 21 and 22 of theMach-Zehnder structure that has birefringence in the other arm portionin the polarization beam splitter section 1 and the polarization beamcombiner section 3 are employed to further perform the refractive indexcontrol of the one arm portion. Therefore, according to the presentpreferred embodiment, there can be provided a polarization dispersioncompensation apparatus, which has a smaller size and a lighter weightthan those of the prior art and is able to make compensation for thepolarization dispersion with a lower loss. Furthermore, by virtue ofremoval of the movable portion, deterioration due to aging can bereduced, and reliability can be improved.

In the aforementioned preferred embodiment, the optical waveguide isheated by means of the heater in the temperature control portions HT1and HT2. However, the present invention is not limited to this, and itis acceptable to control the temperature by cooling the opticalwaveguide by means of a cooling apparatus such as a Peltier device, soas to change the refractive index in the optical waveguide.

Second Preferred Embodiment

FIG. 6 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to thesecond preferred embodiment of the present invention, and FIG. 7 is aplan view showing a detailed construction of the polarization controller4 a of FIG. 6. The polarization dispersion compensation apparatus ofthis preferred embodiment is characterized in that a polarizationcontroller 4 a formed on the optical waveguide 21 shown in FIG. 7 isprovided in place of the bulk type polarization controller 4 as comparedwith the first preferred embodiment shown in FIG. 1, and this point ofdifference will be described below.

Referring to FIG. 7, the polarization controller 4 a is an apparatuswhich has been known to those skilled in the art, and is constituted byincluding a quarter-wavelength plate 43 and a half-wavelength plate 44.A strip-shaped grounding electrode 60 is formed on an optical waveguide21 on an optical waveguide substrate 10 over the length in the opticalsignal propagation direction of the quarter-wavelength plate 43 and thehalf-wavelength plate 44. Moreover, in the quarter-wavelength plate 43,a pair of electrodes 61 and 62 each having a quarter wavelength isformed on both sides in the horizontal direction of the opticalwaveguide 21 on the optical waveguide substrate 10. By adjusting a DCvoltage to be applied to a pair of electrodes 61 and 62, thepolarization angle of the optical signal that propagates through theoptical waveguide 21 can be rotated. Further, in the half-wavelengthplate 44, a pair of electrodes 63 and 64 each having a half wavelengthis formed on both sides in the horizontal direction of the opticalwaveguide 21 on the optical waveguide substrate 10. By adjusting a DCvoltage to be applied to a pair of electrodes 63 and 64, thepolarization angle of the optical signal that propagates through theoptical waveguide 21 can be rotated.

In the polarization controller 4 a constituted as above, thepolarization angle of rotation of the optical signal that propagatesthrough the wavelength plates 43 and 44 is adjusted by adjusting thelevels of the DC voltage applied to the electrodes 61 and 62 of thequarter-wavelength plate 43 and the DC voltage applied to the electrodes63 and 64 of the half-wavelength plate 44. By this operation, thepolarization state of the optical signal is controlled so that thepolarization axis of the optical signal that propagates through theoptical waveguide 21 substantially coincides with the optical axis ofthe optical waveguide 21. By employing the polarization controller 4 ain place of the polarization controller 4, all the elements can beformed on the optical waveguide substrate 10, allowing achievement offurther downsizing.

Moreover, in a manner similar to that of the first preferred embodiment,a polarization dispersion compensation apparatus can be constituted byincluding the polarization controller 4 a, the polarization beamsplitter section 1, the optical delay circuit section 2 and thepolarization beam combiner section 3. That is, the polarizationdispersion between the TE wave and the TM wave can be adjusted inaccordance with the environmental change of the optical fiber cable fortransmission connected at the preceding stage of this polarizationdispersion compensation apparatus so as to consistently makecompensation by adjusting the temperature control portions HT1 and HT2in a similar manner. Therefore, according to the present preferredembodiment, there can be provided a polarization dispersion compensationapparatus, which has a smaller size and a lighter weight than those ofthe prior art and is able to make compensation for the polarizationdispersion with a lower loss. Furthermore, by virtue of removal of themovable portion, deterioration due to aging can be reduced, andreliability can be improved.

Third Preferred Embodiment

FIG. 8 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to thethird preferred embodiment of the present invention. The polarizationdispersion compensation apparatus of this third preferred embodimentdiffers from the polarization dispersion compensation apparatus shown inFIG. 6 in the following points.

(a) A polarization beam splitter section 1 a that has an electric fieldcontrol portion EF1 is provided in place of the polarization beamsplitter section 1 that has the temperature control portion HT1.

(b) A polarization beam combiner section 3 a that has an electric fieldcontrol portion EF2 is provided in place of the polarization beamcombiner section 3 that has the temperature control portion HT2.

These points of difference will be described below. FIG. 9 is a planview showing a detailed construction of the electric field controlportion EF1 of FIG. 8, and FIG. 10 is a longitudinal sectional viewshowing a detailed construction of the electric field control portionEF1 of FIG. 8. Referring to FIG. 10, an optical waveguide substrate 10is formed by forming a quartz film 12 on a silicon substrate 11, and acore 21 c of the optical waveguide 21 is formed by doping the quartzfilm 12 internally with an impurity. A chrome mask pattern is formed onthe surface of the silicon substrate 11 by the photoengraving process,and thereafter, a groove 15 is formed by etching the silicon substrate11 located just below the core 21 c with an etchant of a volumetricratio of hydrofluoric acid:nitric acid:acetic acid=1:4:3. Subsequently,an electrode 74, which is a thin metallic wire, is inserted in thelongitudinal direction of the core 21 c so as to be brought in contactwith the quartz film 12. On the other hand, an electrode 73 made of athin metallic wire is mounted on a portion just above the core 21 cwhile being bonded in the longitudinal direction of the core 21 c.Further, a chrome mask pattern is formed on the surface of the quartzfilm 12 by a photoengraving process, and thereafter, grooves 13 and 14are formed by etching both sides in the horizontal direction of the core21 c in the direction of thickness of the quartz film 12 by a reactiveion-etching method. Subsequently, electrodes 71 and 72 each made of thinmetallic wire are mounted so as to be positioned on both sides of thecore 21 c in the horizontal direction of the core 21 c and inserted inthe longitudinal direction of the core 21 c.

Further, by irradiating the core 21 c of the optical waveguide 21 of theelectric field control portion EF1 with ultraviolet rays by means of,for example, an ArF laser apparatus 50 provided with an ArF excimerlaser, a polling process for ultraviolet ray excitation is performed.According to this ultraviolet ray excitation polling, a DC voltage wasapplied so that the electric field intensity on the core 21 c of theoptical waveguide 21 became about 105 V/cm with respect to theelectrodes 71, 72, 73 and 74 while irradiating the optical waveguide 21before the film formation of the grounding electrode with theultraviolet rays of the ArF excimer laser that has an energy of 50mJ/cm² and a wavelength of 193 nm. Subsequently, a grounding electrodefilm was formed, and a voltage control section EF1, or an opticalwaveguide type polarization controller, was formed. In this case, theglass material of the core 21 c originally has no electro-optic effectsince it has a centrosymmetry. However, the electric field control ofthe refractive index can be performed when the polling process isperformed, and polarization control can be performed. Although thepolling process for ultraviolet ray excitation is performed in thepresent preferred embodiment, it is acceptable to perform thermalexcitation polling for applying an electric field while heating theoptical waveguide 21.

When a DC voltage is applied to the electrodes 71, 72, 73 and 74 afterthe polling process is performed, different refractive index changes arecaused to the TE wave and the TM wave. This means that an opticalwaveguide type element corresponding to the quarter-wavelength plate andthe half-wavelength plate is provided by virtue of the enabledbirefringence control, and the adjustment of the value of the DC voltageapplied by the control signal corresponds to the adjustment of thepolarization rotation angle of the polarization controller 4 of thefirst preferred embodiment. Therefore, the electric field controlportion EF1 controls the refractive index of the optical signal thatpropagates through the optical waveguide 21 by controlling the DCvoltage applied to the electrodes 71 to 74 provided in the opticalwaveguide 21 in a manner similar to that of the temperature controlportion HT1. Moreover, the electric field control portion EF2 of thepolarization beam combiner section 3 is formed in a manner similar tothat of the electric field control portion EF1.

In the polarization controller 4 a constituted as above, thepolarization dispersion between the TE wave and the TM wave can beadjusted in accordance with the environmental change of the opticalfiber cable for transmission connected at the preceding stage of thispolarization dispersion compensation apparatus so as to consistentlymake compensation by adjusting the electric field control portions EF1and EF2 in a similar manner. Therefore, according to the presentpreferred embodiment, there can be provided a polarization dispersioncompensation apparatus, which has a smaller size and a lighter weightthan those of the prior art and is able to make compensation for thepolarization dispersion with a lower loss. Furthermore, by virtue ofremoval of the movable portion, deterioration due to aging can bereduced, and reliability can be improved. Moreover, since the refractiveindex is controlled by means of the electric field control portions EF1and EF2, the switching characteristic is allowed to have a higher speed.

Fourth Preferred Embodiment

FIG. 11 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to thefourth preferred embodiment of the present invention. The polarizationdispersion compensation apparatus of this fourth preferred embodiment ischaracterized in that an optical delay circuit section 2 a isconstituted by providing a temperature control portion HT3 in theoptical waveguide 22 of the optical delay circuit section 2 as comparedwith the first preferred embodiment shown in FIG. 1. This point ofdifference will be described below.

The temperature control portion HT3 is formed in a manner similar tothat of the temperature control portions HT1 and HT2 and able to finelyadjust the polarization dispersion value corresponding to the groupdelay time difference between the TE wave and the TM wave by finelyadjusting the refractive index of the optical waveguide 22 with theelectric power applied to the heater of the temperature control portionHT3 changed.

According to the fourth preferred embodiment constituted as above, thereis such a particular advantageous effect that the polarizationdispersion value corresponding to the group delay time differencebetween the TE wave and the TM wave can be finely adjusted by finelyadjusting the refractive index of the optical waveguide 22 in additionto the operation and advantageous effects of the first preferredembodiment.

Fifth Preferred Embodiment

FIG. 12 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to thefifth preferred embodiment of the present invention. The polarizationdispersion compensation apparatus of this fifth preferred embodiment ischaracterized in that an optical delay circuit section 2 b isconstituted by providing an electric field control portion EF3 in theoptical waveguide 22 of the optical delay circuit section 2 as comparedwith the third preferred embodiment shown in FIG. 8. This point ofdifference will be described below.

The electric field control portion EF3 is formed in a manner similar tothat of the electric field control portions EF1 and EF2 and is able tofinely adjust the polarization dispersion value corresponding to thegroup delay time difference between the TE wave and the TM wave byfinely adjusting the refractive index of the optical waveguide 22 withthe DC voltage applied to the electrodes of the electric field controlportion EF3 changed.

According to the fifth preferred embodiment constituted as above, thereis such a particular advantageous effect that the polarizationdispersion value corresponding to the group delay time differencebetween the TE wave and the TM wave can be finely adjusted by finelyadjusting the refractive index of the optical waveguide 22 in additionto the operation and advantageous effects of the third preferredembodiment.

Sixth Preferred Embodiment

FIG. 13 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to thesixth preferred embodiment of the present invention. The polarizationdispersion compensation apparatus of this sixth preferred embodimentdiffers from the first preferred embodiment shown in FIG. 1 in thefollowing points.

(a) A birefringence portion BF11 is formed in the mutually adjacentportions of the optical waveguides 21 and 22 of a directional couplerDC11 in place of the polarization beam splitter section 1, and apolarization beam splitter section 1 c constituted by forming atemperature control portion HT1 is provided.

(b) A birefringence portion BF12 is formed in the mutually adjacentportions of the optical waveguides 21 and 22 of a directional couplerDC12 in place of the polarized polarization beam combiner section 3, anda polarization beam combiner section 3 c constituted by forming atemperature control portion HT2 is provided.

These points of difference will be described below. The polarizationdispersion compensation apparatus of this preferred embodiment isconstituted by including a polarization controller 4, a polarizationbeam splitter section 1 c, an optical delay circuit section 2 and apolarization beam combiner section 3 c.

On the optical waveguide substrate 10, two optical waveguides 21 and 22are formed to be adjacent to each other so that optical signals arecombined with each other in two portions, and these two portions becomethe 3-dB directional couplers DC11 and DC12. In this case, thedirectional coupler DC11 has two input ports P51 and P52 as well as twooutput ports P53 and P54, and the directional coupler DC12 has two inputports P61 and P62 as well as two output ports P63 and P64.

In the polarization beam splitter section 1 c, the birefringence portionBF11 is formed by irradiating the mutually adjacent two opticalwaveguides 21 and 22 of the directional coupler DC11 with ultravioletrays, which have an energy of 50 mJ/cm² and a wavelength of 193 nm andis subjected to pulse intensity modulation at a frequency of 60 Hz forfive minutes by means of an ArF laser apparatus 50 provided with an ArFexcimer laser. This birefringence portion BF11 causes birefringence inthe optical signal that propagates through the optical waveguide 22 thatis the other arm portion. Further, in the formation position of thisbirefringence portion BF11, a temperature control portion HT1 is formed.The temperature control portion HT1 is provided to control therefractive index of the optical signal that propagates through theoptical waveguides 21 and 22 by controlling the temperature of theoptical waveguides 21 and 22 by heating.

A polarization beam splitter section 1 c is provided with thetemperature control portion HT1 and the birefringence portion BF11,which are thus formed. By virtue of the polarization beam splittersection 1 c, the inputted optical signal can be split into the TE waveand the TM wave, which are polarized components perpendicular to eachother, in a manner similar to that of the polarization beam splittersection 1. That is, the TE wave is outputted from the output port P53,while the TM wave is outputted from the output port P54.

Moreover, in a manner similar to that of the polarization beam splittersection 1 c, there is formed a polarization beam combiner section 3 cprovided with a birefringence portion BF12 and a temperature controlportion HT2 at the subsequent stage of the optical delay circuit section2. The TE wave and the TM wave split by the polarization beam splittersection 1 c are combined by this polarization beam combiner section 3 c,and an optical signal obtained after the combining is outputted from theoutput port P63.

As described above, the polarization dispersion between the TE wave andthe TM wave can be adjusted in accordance with the environmental changeof the optical fiber cable for transmission connected at the precedingstage of this polarization dispersion compensation apparatus so as toconsistently make compensation by adjusting the temperature controlportions HT1 and HT2 in a similar manner.

As compared with the prior art example, the elements, which constitutethe apparatus, can further be downsized and made to have a lower losswith the construction in which the optical waveguides 21 and 22 that hasbirefringence in the polarization beam splitter 1 c and the polarizationbeam combiner section 3 c are employed to further perform the refractiveindex control. Therefore, according to the present preferred embodiment,there can be provided a polarization dispersion compensation apparatus,which has a smaller size and a lighter weight than those of the priorart and is able to make compensation for the polarization dispersionwith a lower loss. Furthermore, by virtue of removal of the movableportion, deterioration due to aging can be reduced, and reliability canbe improved.

In the aforementioned sixth preferred embodiment, the temperaturecontrol portions HT1 and HT2 are formed. However, the present inventionis not limited to this, and it is acceptable to form electric fieldcontrol portions EF1 and EF2 in place of the temperature controlportions HT1 and HT2.

Seventh Preferred Embodiment

FIG. 14 is a plan view showing a construction of an optical fiber cabletype polarization dispersion compensation apparatus according to theseventh preferred embodiment of the present invention. The polarizationdispersion compensation apparatus of this seventh preferred embodimentis characterized in that the optical waveguides 21 and 22 in thepolarization beam splitter section 1, the optical delay circuit section2 and the polarization beam combiner section 3 are constituted byincluding optical fiber cables 23, 24, 25, 26, 27 and 28 as comparedwith the first preferred embodiment shown in FIG. 1.

Referring to FIG. 14, the polarization dispersion compensation apparatusof this preferred embodiment is constituted by including a polarizationcontroller 4, a polarization beam splitter section 1 d, an optical delaycircuit section 2 d and a polarization beam combiner section 3 d. Theoptical fiber cable 20 located on the input side where the polarizationcontroller 4 is provided is connected with the optical fiber cable 23 ofthe polarization beam splitter section 1 d via an optical fiberconnector 20CA.

The two optical fiber cables 23 and 24 of the polarization beam splittersection 1 d are formed to be adjacent to each other in two places, andthe adjacent portions constitute 3-dB directional couplers DC11 andDC12, respectively. A resistive terminator 93 is connected with theinput terminal of the optical fiber cable 24 of the directional couplerDC11. Moreover, the two optical fiber cables 23 and 24 located betweenthe two directional couplers DC11 and DC12 constitute two arm portionsof a symmetric Mach-Zehnder interferometer, respectively. A temperaturecontrol portion HT11 is formed in one arm portion, while a birefringenceportion BF11 formed of an ArF laser apparatus 50 is provided in theother arm portion.

Further, the optical fiber cable 23 located on the output side of thedirectional coupler DC12 is connected with an optical fiber cable 25 ofthe optical delay circuit section 2 d via an optical fiber connector23C. The optical fiber cable 24 located on the output side of thedirectional coupler DC12 is connected with an optical fiber cable 26 ofthe optical delay circuit section 2 d via an optical fiber connector24C. In this case, the optical fiber cable 25 and the optical fibercable 26 are formed so as to have lengths different from each other.Furthermore, the terminal of the optical fiber cable 25 is connectedwith an optical fiber cable 27 of the polarization beam combiner section3 d via an optical fiber connector 25C. The terminal of the opticalfiber cable 26 is connected with an optical fiber cable 28 of thepolarization beam combiner section 3 d via an optical fiber connector26C. It is to be noted that a resistive terminator 28 is provided at theterminal of the optical fiber cable 28.

The two optical fiber cables 27 and 28 of the polarization beam combinersection 3 d are formed to be adjacent to each other in two places, andthe adjacent portions constitute 3-dB directional couplers DC13 andDC14, respectively. The two optical fiber cables 27 and 28 locatedbetween the two directional couplers DC13 and DC14 constitute two armportions of a symmetric Mach-Zehnder interferometer, respectively. Atemperature control portion HT12 is formed in one arm portion, while abirefringence portion BF12 formed of an ArF laser apparatus 50 isprovided in the other arm portion.

By executing the adjustment of the electric power applied to thetemperature control portion HT12 of the polarization beam combinersection 3 d in the polarization dispersion compensation apparatusconstituted as above in a manner similar to that of the adjustmentmethod of the electric power applied to the temperature control portionHT11 of the polarization beam splitter section 1 d, it can be controlledso that an optical signal is consistently outputted from the terminalportion of the optical fiber cable 27. Therefore, the polarizationdispersion between the TE wave and the TM wave can be adjusted inaccordance with the environmental change of the optical fiber cable fortransmission connected at the preceding stage of this polarizationdispersion compensation apparatus so as to consistently makecompensation by adjusting the temperature control portions HT11 and HT12in a similar manner.

As compared with the prior art example, the elements, which constitutethe apparatus, can be downsized and made to have a lower loss with theconstruction in which the optical fiber cables of the Mach-Zehnderstructure that has birefringence are employed in the other arm portionin the polarization beam splitter 1 d and the polarization beam combinersection 3 d to further perform the refractive index control of one armportion. Therefore, according to the present preferred embodiment, therecan be provided a polarization dispersion compensation apparatus, whichhas a smaller size and a lighter weight than those of the prior art andis able to make compensation for the polarization dispersion with alower loss. Furthermore, by virtue of removal of the movable portion,deterioration due to aging can be reduced, and reliability can beimproved.

In the aforementioned seventh preferred embodiment, the temperaturecontrol portions HT11 and HT12 are formed. However, the presentinvention is not limited to this, and it is acceptable to form electricfield control portions EF1 and EF2 in place of the temperature controlportions HT11 and HT12.

Eighth Preferred Embodiment

FIG. 15 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to theeighth preferred embodiment of the present invention.

Referring to FIG. 15, an input channel optical waveguide 211, an opticalwaveguide type polarization beam splitter element 212, a pair of opticalwaveguides 213 a and 213 b, a variable branching ratio optical coupler214, a pair of optical delay lines 215 a and 215 b, an optical waveguidetype polarization beam combiner element 216, output channel opticalwaveguides 217 a and 217 b are successively formed on a siliconsubstrate 210. Further, polarization change means 218 a and 218 b areformed in each of a pair of optical waveguides 213 a and 213 b and apair of optical delay lines 215 a and 215 b. Moreover, phase adjustmentmeans 219 a and 219 b for adjusting the relative phase difference areformed in a pair of optical waveguides 213 a and 213 b. In this case,one input port of the optical waveguide type polarization beam splitterelement 212 is optically connected with the input channel opticalwaveguide 211, and two input ports of the variable branching ratiooptical coupler 214 are optically connected with the two output ports ofthe optical waveguide type polarization beam splitter element 212 via apair of optical waveguides 213 a and 213 b. Moreover, a pair of opticaldelay lines 215 a and 215 b has one end optically connected with the twooutput ports of the variable branching ratio optical coupler 214 and theother end optically connected with the two input ports of the opticalwaveguide type polarization beam combiner element 216. Two output portsof the optical waveguide type polarization beam combiner element 216 areoptically connected with the output channel optical waveguides 217 a and217 b.

In this case, the optical waveguide type polarization beam splitterelement 212 is constituted in a manner similar to that of thepolarization beam splitter sections 1, 1 a and 1 c, and the opticalwaveguide type polarization beam combiner element 216 is constituted ina manner similar to that of the polarization beam combiner sections 3, 3a and 3 c. The optical waveguide type polarization beam combiner element216 is a so-called 90 degrees hybrid circuit, and the terminal portionsof the output channel optical waveguides 217 a and 217 b opticallyconnected with the two output ports of the optical waveguide typepolarization beam combiner element 216 are referred to as a first outputport 217 ap that outputs a combined optical signal and a second outputport 217 b that outputs no combined optical signal, respectively.

The polarization change means 218 a and 218 b are constituted byinserting a half-wavelength plate in the optical waveguide, and thephase adjustment means 219 a and 219 b are constituted by forming atemperature control portion of a thin film heater or the like in theoptical waveguide. Further, the variable branching ratio optical coupler214 can be constituted by providing a Mach-Zehnder type interferometerwith two optical output couplers having two inputs and two outputs, andtwo-arm optical waveguides for connecting them and forming a temperaturecontrol portion of a thin film heater or the like on one arm opticalwaveguide. In this case, if the optical path length difference betweenthe two arm optical waveguides is adjusted by controlling thetemperature control portion, then the branching ratio from the twoinputs of one optical coupler to the two outputs of the other opticalcoupler can be changed.

The polarization change means 218 a, which is formed in the opticalwaveguide 213 a in this preferred embodiment, may be formed in theoptical waveguide 213 b. The phase adjustment means 219 a and 219 b,which are formed respectively in the optical waveguides 213 a and 213 b,may be provided in either one of the optical waveguides 213 a and 213 b.Moreover, it is acceptable to exchange the positions of the polarizationchange means 218 a and the phase adjustment means 219 a. Furthermore,the polarization change means 218 b, which is formed in the opticaldelay line 215 b in this case, may be formed in the optical delay line215 a. It is also acceptable to exchange the positions of thepolarization change means 218 b and the optical delay line 215 b.

In the polarization dispersion compensation apparatus constituted asabove, the phase adjustment means 219 a and 219 b and the variablebranching ratio optical coupler 214 operate as a polarization controller300, and the optical delay lines 215 a and 215 b operate as a variabledelay line, constituting a polarization dispersion compensation circuitas a whole. With this arrangement, this polarization dispersioncompensation circuit can make compensation for the primary polarizationdispersion in the optical transmission line.

A polarization dispersion compensation control method according to thepresent invention with this polarization dispersion compensationapparatus will be described below.

As described above, the linearly polarized waves of the TM wave and theTE wave are incident onto the optical delay lines 215 a and 215 b.Accordingly, the linearly polarized waves perpendicular to each otherare combined in the optical waveguide type polarization combiner element216, and all the optical output signals should be outputted to the firstoutput port 217 ap. Therefore, by controlling the phase adjustment means219 a and 219 b and the variable branching ratio coupler 214 so that thesignal level of the optical signal outputted from the first output port217 ap becomes the maximum or the signal level of the optical signaloutputted from the second output port 217 bp becomes the minimum, thepolarization in the optical delay lines 215 a and 215 b can be adjustedto the optimum state. By controlling the amount of delay in the opticaldelay lines 215 a and 215 b in a manner that the signal component of thefrequency being half the bit rate frequency is extracted from thereceived signal obtained by demodulating the optical signal received atthe first output port 217 ap and the signal level of this signalcomponent becomes the maximum, then the optimum state is obtained. Thepolarization controller 300 and the optical delay lines 215 a and 215 bcan be individually adjusted by using the method as described above, andtherefore, realistic control can be performed.

Although the optical signal is outputted from the first output portaccording to the setting of this preferred embodiment, it is alsopossible to adopt the setting of outputting the optical signal from thesecond output port. For example, if the refractive index of the armportion of the optical waveguide type polarization beam combiner element216 constituted by including the Mach-Zehnder interferometer is adjustedby the heater to shift the phase of the optical signal only by 180degrees, then the setting of outputting the optical signal from thesecond output port is achieved. In this case, by controlling thepolarization controller so that the signal level of the optical signaloutputted from the second output port becomes the maximum or the signallevel of the optical signal outputted from the first output port becomesthe minimum, the polarization in the optical delay lines can be adjustedto the optimum state.

As described above, according to the prior art disclosed in the thirdprior art document, the control of compensating polarization dispersionwas difficult. However, according to the present preferred embodiment,the control of compensating the polarization dispersion can beremarkably simplified as compared with the prior art.

Ninth Preferred Embodiment

FIG. 16 is a plan view showing a construction of an optical waveguidetype polarization dispersion compensation apparatus according to theninth preferred embodiment of the present invention. The polarizationdispersion compensation apparatus of this preferred embodiment ischaracterized in that another polarization controller 301 is insertedbetween the polarization controller 300 and the optical delay lines 215a and 215 b as compared with the eighth preferred embodiment shown inFIG. 15. This point of difference will be described below.

Another polarization controller 301 is constituted by including phaseadjustment means 229 a and 229 b formed in arm portion opticalwaveguides 223 a and 223 b optically connected with two output ports ofa variable branching ratio optical coupler 214 and a variable branchingratio optical coupler 224 optically connected at the subsequent stage.In the polarization dispersion compensation apparatus constituted asabove, the restriction on the adjustable amount of phase can bealleviated as compared with the eighth preferred embodiment that hasonly one polarization controller 300.

Even in this polarization dispersion compensation apparatus, bycontrolling the phase adjustment means 219 a, 219 b, 229 a and 229 b andthe variable branching ratio coupler 214 so that the signal level of theoptical signal outputted from the first output port 217 ap becomes themaximum or the signal level of the optical signal outputted from thesecond output port 217 bp becomes the minimum in a manner similar tothat of the eighth preferred embodiment, the polarization in the opticaldelay lines 215 a and 215 b can be adjusted to the optimum state.Therefore, difficulties in controlling the polarization controllers 300and 301 are alleviated.

As described above, the control of compensating polarization dispersionwas difficult according to the prior art disclosed in the third priorart document. However, according to the present preferred embodiment,the control of compensating polarization dispersion can be remarkablysimplified as compared with the prior art.

The aforementioned eighth and ninth preferred embodiments describes themethod for controlling the amount of polarization dispersioncompensation by changing the amount of delay of the optical delay lines215 a and 215 b. For example, even in the polarization dispersioncompensation apparatus of a type such that the amount of delay ischanged by adjusting the branching ratio of the optical signals in theoptical delay lines 215 a and 215 b by means of the variable branchingratio couplers 214 and 224 or the like at the preceding stage of theoptical delay lines 215 a and 215 b with the amount of delay fixed, themethod for executing control so that the signal level of the opticalsignal outputted from the first output port 217 ap becomes the maximumor the signal level of the optical signal outputted from the secondoutput port 217 bp becomes the minimum is extremely useful.

Modified Preferred Embodiments

FIG. 17 is a plan view showing a construction of part of a polarizationbeam splitter section 1 for an optical waveguide type polarizationdispersion compensation apparatus according to the first modifiedpreferred embodiment of the present invention. In the polarization beamsplitter section 1 of FIG. 1, the temperature control portion HT1 andthe birefringence portion BF1 are formed the arm portions. However, asshown in FIG. 17, it is acceptable to form the temperature controlportion HT1 and the birefringence portion BF1 in concatenation in onearm portion of the polarization beam splitter section 1.

FIG. 18 is a plan view showing a construction of part of a polarizationbeam splitter section 1 for an optical waveguide type polarizationdispersion compensation apparatus according to the second modifiedpreferred embodiment of the present invention. As shown in FIG. 18, itis acceptable to form the temperature control portion HT1 and thebirefringence portion BF1 in a superposing manner in one arm portion ofthe polarization beam splitter section 1.

FIG. 19 is a plan view showing a construction of part of a polarizationbeam splitter section 1 for an optical waveguide type polarizationdispersion compensation apparatus according to the third modifiedpreferred embodiment of the present invention. As shown in FIG. 19, itis acceptable to form the birefringence portion BF1 in one arm portionof the polarization beam splitter section 1 and form the temperaturecontrol portion HT1 in the other arm portion.

FIG. 20 is a plan view showing a construction of part of a polarizationbeam combiner section 3 for an optical waveguide type polarizationdispersion compensation apparatus according to the fourth modifiedpreferred embodiment of the present invention. In the polarization beamcombiner section 3 of FIG. 1, the temperature control portion HT2 andthe birefringence portion BF2 are formed in the arm portions. However,as shown in FIG. 20, it is acceptable to form the temperature controlportion HT2 and the birefringence portion BF2 in concatenation in onearm portion of the polarization beam combiner section 3.

FIG. 21 is a plan view showing a construction of part of a polarizationbeam combiner section 3 for an optical waveguide type polarizationdispersion compensation apparatus according to the fifth modifiedpreferred embodiment of the present invention. As shown in FIG. 21, itis acceptable to form the temperature control portion HT2 and thebirefringence portion BF2 in a superposing manner in one arm portion ofthe polarization beam combiner section 3.

FIG. 22 is a plan view showing a construction of part of a polarizationbeam combiner section 3 for an optical waveguide type polarizationdispersion compensation apparatus according to the sixth modifiedpreferred embodiment of the present invention. As shown in FIG. 22, itis also acceptable to form the birefringence portion BF2 in one armportion of the polarization beam combiner section 3 and form thetemperature control portion HT2 in the other arm portion.

That is, as shown in FIGS. 17 to 22, in the polarization beam splittersection 1 or the polarization beam combiner section 3, it is acceptableto provide the temperature control portion and the birefringence portionin an identical arm portion or superpose them on each other in anidentical arm portion or exchange the positions of the temperaturecontrol portion and the birefringence portion.

In the aforementioned preferred embodiments, the birefringence portionis formed on the optical waveguide. However, the optical waveguideformed on the optical waveguide substrate innately has birefringenceascribed to a thermal expansion difference between the optical waveguidesubstrate and the optical waveguide. Even in the above case, it isacceptable to form a polarization beam splitter section and apolarization beam combiner section by changing the birefringence bymeans of irradiation with an ArF excimer laser or other means.

INDUSTRIAL APPLICABILITY

As described above in detail, according to one aspect of the presentinvention, there is provided a polarization dispersion compensationapparatus including:

polarization control means for controlling a polarization state of aninputted optical signal so that a polarization axis of the opticalsignal substantially coincides with an optical axis of an opticaltransmission line;

polarization beam splitter means for splitting an optical signaloutputted from the polarization control means, and outputting opticalsignals of two polarized components perpendicular to each other;

optical delay means including two optical transmission lines of lengthsdifferent from each other, and causing a difference in delay between thetwo polarized components of the optical signal outputted from thepolarization beam splitter means; and

polarization beam combiner means for combining the two polarizedcomponents of the optical signal outputted from the optical delay means,and outputting a combined optical signal,

wherein the polarization beam splitter means includes a symmetricMach-Zehnder interferometer having optical transmission lines of firstand second arm portions, and at least one of the first and second armportions includes first refractive index control means for controlling arefractive index of the optical signal propagating through the opticaltransmission line of the arm portion and first birefringence means forcausing birefringence in the optical signal propagating through theoptical transmission line of the arm portion,

wherein the polarization beam combiner means includes a symmetricMach-Zehnder interferometer having optical transmission lines of thirdand fourth arm portions, and at least one of the third and fourth armportions includes second refractive index control means for controllinga refractive index of the optical signal propagating through the opticaltransmission line of the arm portion and second birefringence means forcausing birefringence in the optical signal propagating through theoptical transmission line of the arm portion.

Therefore, the polarization dispersion between the TE wave and the TMwave can be adjusted in accordance with the environmental change of theoptical fiber cable for transmission connected at the preceding stage ofthis polarization dispersion compensation apparatus so as toconsistently make compensation by adjusting the first and secondrefractive index control means in a similar manner. As compared with theprior art example, the elements, which constitute the apparatus, can bedownsized and made to have a lower loss with the construction in whichthe optical transmission lines of the Mach-Zehnder structure that hasbirefringence in the other arm portion in the polarization beam splittermeans and the polarization beam combiner means are employed to executethe refractive index control of the one arm portion. Therefore,according to the present invention, there can be provided a polarizationdispersion compensation apparatus, which has a smaller size and alighter weight than those of the prior art and is able to makecompensation for the polarization dispersion with a lower loss.Furthermore, by virtue of removal of the movable portion, deteriorationdue to aging can be reduced, and reliability can be improved.

Moreover, according to another aspect of the present invention, there isprovided a polarization dispersion compensation apparatus including:

polarization control means for controlling a polarization state of aninputted optical signal so that a polarization axis of the opticalsignal substantially coincides with an optical axis of an opticaltransmission line;

polarization beam splitter means for splitting an optical signaloutputted from the polarization control means, and outputting opticalsignals of two polarized components perpendicular to each other;

optical delay means including two optical transmission lines of lengthsdifferent from each other, and causing a difference in delay between thetwo polarized components of the optical signal outputted from thepolarization beam splitter means; and

polarization beam combiner means for combining the two polarizedcomponents of an optical signal outputted from the optical delay means,and outputting a resulting combined optical signal,

wherein the polarization beam splitter means including a fifthdirectional coupler having mutually adjacent two optical transmissionlines, distributing the inputted optical signal into two opticalsignals, and outputting the distributed two optical signals,

wherein the mutually adjacent two optical transmission lines of thefifth directional coupler includes fourth refractive index control meansfor controlling a refractive index of optical signals propagatingthrough the two optical transmission lines and third birefringence meansfor causing birefringence in the optical signals propagating through thetwo optical transmission lines,

wherein the polarization beam combiner means includes a sixthdirectional coupler having mutually adjacent two optical transmissionlines, distributing the inputted optical signal into two opticalsignals, and outputting the distributed two optical signals, and

wherein the mutually adjacent two optical transmission lines of thesixth directional coupler includes fifth refractive index control meansfor controlling a refractive index of optical signals propagatingthrough the two optical transmission lines and fourth birefringencemeans for causing birefringence in the optical signals propagatingthrough the two optical transmission lines.

Therefore, the polarization dispersion between the TE wave and the TMwave can be adjusted in accordance with the environmental change of theoptical fiber cable for transmission connected at the preceding stage ofthis polarization dispersion compensation apparatus so as toconsistently make compensation by adjusting the first and secondrefractive index control means in a similar manner. As compared with theprior art example, the elements, which constitute the apparatus, can bedownsized and made to have a lower loss with the construction in whichthe optical transmission lines that have birefringence in thepolarization beam splitter means and the polarization beam combinermeans are employed to execute the refractive index control. Therefore,according to the present invention, there can be provided a polarizationdispersion compensation apparatus, which has a smaller size and alighter weight than those of the prior art and is able to makecompensation for the polarization dispersion with a lower loss.Furthermore, by virtue of removal of the movable portion, deteriorationdue to aging can be reduced, and reliability can be improved.

Furthermore, according to a further aspect of the present invention,there is provided a polarization dispersion compensation apparatusincluding:

polarization beam splitter means for splitting and outputting opticalsignals of two polarized components perpendicular to each other;

polarization control means including phase adjustment means and avariable branching ratio coupler and controlling a polarization state ofthe optical signal;

a pair of optical delay means for delaying the optical signal after thepolarization beam splitting; and

polarization beam combiner means having first and second output ports,combining the two polarized components of the optical signal outputtedfrom the optical delay means, and outputting a resulting combinedoptical signal through the first output port,

wherein the polarization beam splitter means and the polarization beamcombiner means is of a symmetric Mach-Zehnder interferometer havingbirefringence at least in one arm portion, and

wherein the phase adjustment means and the variable branching ratiocoupler is controlled so that a level of a signal outputted from thefirst output port of the polarization beam combiner means becomes themaximum or so that a level of a signal outputted from the second outputport of the polarization beam combiner means becomes the minimum.

The control of compensating polarization dispersion was difficultaccording to the prior art. However, according to the present invention,the control of compensating polarization dispersion can be remarkablysimplified as compared with the prior art.

1. A polarization dispersion compensation apparatus comprising:polarization control means for controlling a polarization state of anoptical signal so that a polarization axis of the optical signalsubstantially coincides with an optical axis of an optical transmissionline; polarization beam splitter means for splitting an optical signaloutput from said polarization control means, and outputting opticalsignals having polarized components perpendicular to each other; opticaldelay means including two optical transmission lines having lengthsdifferent from each other, and causing a difference in delay between thepolarized components of the optical signal output from said polarizationbeam splitter means; and polarization beam combiner means for combiningthe polarized components of the optical signal output from said opticaldelay means, and outputting a combined optical signal, wherein saidpolarization beam splitter means comprises a first symmetricMach-Zehnder interferometer having optical transmission lines in firstand second arm portions, at least one of the first and second armportions comprising first refractive index control means for controllingrefractive index for the optical signal propagating through the opticaltransmission line of the arm portion and first birefringence means forcausing birefringence of the optical signal propagating through theoptical transmission line of the arm portion, said polarization beamcombiner means comprises a second symmetric Mach-Zehnder interferometerhaving optical transmission lines in third and fourth arm portions, andat least one of the third and fourth arm portions comprising secondrefractive index control means for controlling refractive index for theoptical signal propagating through the optical transmission line of thearm portion and second birefringence means for causing birefringence inthe optical signal propagating through the optical transmission line ofthe arm portion.
 2. The polarization dispersion compensation apparatusaccording to claim 1, wherein the polarization beam splitter meanscomprises: a first directional coupler for distributing an opticalsignal output from said polarization control means as two opticalsignals, and outputting the two optical signals distributed, the opticaltransmission line of the first arm portion propagating a first oneoptical signal of the two optical signals distributed by said firstdirectional coupler, and the optical transmission line of the second armportion propagating a second optical signal of the two optical signalsdistributed by said first directional coupler; and a second directionalcoupler for combining the optical signal propagating through the opticaltransmission line of the first arm portion with the optical signalpropagating through the optical transmission line of the second armportion and thereafter distributing a resulting combined optical signalas two optical signals, and outputting the two optical signalsdistributed; and the polarization beam combiner means comprises: a thirddirectional coupler for combining the polarized components of theoptical signal, and thereafter distributing a resulting combined opticalsignal as two optical signals, and outputting the two optical signalsdistributed, the optical transmission line of the third arm portionpropagating a first optical signals of the two optical signalsdistributed by said third directional coupler, and the opticaltransmission line of the fourth arm portion propagating a second opticalsignal of the two optical signals distributed by said third directionalcoupler; and a fourth directional coupler for combining the opticalsignal propagating through the optical transmission line of the thirdarm portion with the optical signal propagating through the opticaltransmission line of the fourth arm portion and thereafter outputting aresulting combined optical signal.
 3. The polarization dispersioncompensation apparatus according to claim 1, wherein the firstrefractive index control means controls temperature of the opticaltransmission line of the arm portion provided with the first refractiveindex control means, to control the refractive index for the opticalsignal propagating through the optical transmission line of the armportion, and the second refractive index control means controlstemperature of the optical transmission line of the arm portion providedwith the second refractive index control means, to control therefractive index for the optical signal propagating through the opticaltransmission line of the arm portion.
 4. The polarization dispersioncompensation apparatus according to claim 1, wherein said firstrefractive index control means controls an electric field applied to theoptical transmission line of an arm portion subjected to a pollingprocess and which includes the first refractive index control means, tocontrol the refractive index for the optical signal propagating throughthe optical transmission line of the arm portion, and said secondrefractive index control means controls an electric field applied to theoptical transmission line of an arm portion subjected to a pollingprocess and which includes the second refractive index control means, tocontrol the refractive index for the optical signal propagating throughthe optical transmission line of said arm portion.
 5. The polarizationdispersion compensation apparatus according to claim 1, wherein saidfirst birefringence means irradiates the optical transmission line ofthe arm portion including the first birefringence means with ultravioletlight, to cause birefringence in the optical signal propagating throughthe optical transmission line of the arm portion, and said secondbirefringence means irradiates the optical transmission link of the armportion including the second birefringence means with ultraviolet light,to cause birefringence in the optical signal propagating through theoptical transmission line of the arm portion.
 6. The polarizationdispersion compensation apparatus according to claim 1, comprising thirdrefractive index control means for controlling temperature of a firstoptical transmission line of the two optical transmission lines of theoptical delay means, to control the refractive index for the opticalsignal propagating through the first optical transmission line.
 7. Thepolarization dispersion compensation apparatus according to claim 1,wherein a first optical transmission line of the two opticaltransmission lines of said optical delay means is subjected to a pollingprocess, and said polarization dispersion compensation apparatus furthercomprises third refractive index control means for controlling anelectric field applied to the first optical transmission line subjectedto the polling process, to control the refractive index for the opticalsignal propagating through the first optical transmission line.
 8. Thepolarization dispersion compensation apparatus according to claim 1,wherein the optical transmission line is an optical waveguide on asubstrate.
 9. The polarization dispersion compensation apparatusaccording to claim 1, wherein the optical transmission line is anoptical fiber cable.
 10. A polarization dispersion compensationapparatus comprising: polarization control means for controlling apolarization state of an optical signal so that a polarization axis ofthe optical signal substantially coincides with an optical axis of anoptical transmission line; polarization beam splitter means forsplitting an optical signal output from said polarization control means,and outputting optical signals having polarized components perpendicularto each other; optical delay means including two optical transmissionlines having lengths different from each other, and causing a differencein delay between the polarized components of the optical signal outputfrom said polarization beam splitter means; and polarization beamcombiner means for combining the polarized components of the opticalsignal output from said optical delay means, and outputting a combinedoptical signal, wherein the polarization beam splitter means comprises afirst directional coupler having, mutually adjacent, two opticaltransmission lines, distributing the optical signal as two opticalsignals, and outputting the two optical signals distributed, themutually adjacent optical transmission lines of said first directionalcoupler comprise first refractive index control means for controllingrefractive index for optical signals propagating through the two opticaltransmission lines and first birefringence means for causingbirefringence in the optical signals propagating through the two opticaltransmission lines, the polarization beam combiner means comprises asecond directional coupler having, mutually adjacent, two opticaltransmission lines, distributing the optical signal as two opticalsignals, and outputting the two optical signals distributed, and the twooptical transmission lines of the second directional coupler comprisessecond refractive index control means for controlling refractive indexfor optical signals propagating through the two optical transmissionlines and second birefringence means for causing birefringence in theoptical signals propagating through the two optical transmission lines.